Conventionally, as a steel sheet for an iron core (core) of a transformer, there is known a grain-oriented electrical steel sheet which exhibits an excellent magnetic property in a specific direction. The grain-oriented electrical steel sheet is a steel sheet in which a crystal orientation is controlled so that an axis of easy magnetization of crystal grains and a rolling direction correspond with each other by a combination of cold rolling and annealing. A core loss of the grain-oriented electrical steel sheet is desirably as low as possible.
There is known a grain-oriented electrical steel sheet in which an insulating film is formed on a surface of the steel sheet (base steel sheet) in which the crystal orientation is controlled in order to reduce the core loss as described above. This insulating film is responsible for a role in giving not only an electrical insulating property but also tension and heat resistance, and further rust resistance and so on to the steel sheet.
Other than this, as a method for reducing the core loss, there is known a magnetic domain control method in which by forming grooves extending in a direction crossing a rolling direction at predetermined intervals along the rolling direction on a surface of a steel sheet, a magnetic domain is subdivided to make the movement of magnetic domain wall easy, thereby reducing the core loss.
As a method of forming the grooves on the surface of the steel sheet, a method of performing chemical etching (Patent Literature 1) and a laser irradiation method (Patent Literatures 2 to 5) and so on are known.
When the grooves are formed by the chemical etching, a cost related to equipment installation becomes huge, while the laser irradiation method makes it possible to relatively easily and stably form the grooves on the surface of the steel sheet. A method of forming the grooves by performing laser irradiation after forming an insulating film is mentioned in Patent Literatures 3 to 5.
However, in this method, a disappearance of the insulating film accompanying the formation of the grooves cannot be avoided. When the insulating film disappears, rust easily occurs. Although it is possible to improve the rust resistance by forming the insulating film after the formation of the grooves, costs increase.